Production of para xylene



4 Sheets-Sheet 1 J- H. ARNOLD PRODUCTION OF' PARA XYLENE Feb. 13, 1951Filed Aug. 26, 1947 ATTORNEYS Feb. 13, 1951 J, H, ARNOLD 2,541,682

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4 Sheets-Sheet 3 X920 Srv J. H. ARNOLD PRODUCTION 0F PARA XYLENE Feb.13, 1951 Filed Aug. 26, 1947 /NvEN TOR Jerome//'no/o BY A A 7' TORNE Ks'J. H. ARNOLD PRODUCTION oF PARA xYLENE 4 Sheets-Sheet 4 Filed Aug. 26,1947 0@ THG FIG. 7

IN VE N TOR er: ATTORNEYS Patented Feb. 13, 1951 OFFICE PRODUCTION OFPARA XYLENE Jerome Howard Arnold, Albany, Calif., assignor to CaliforniaResearch Corporation, San Francisco, Calif., a corporation of DelawareApplication August 26, 1947, Serial No. 770,587

, Claims. l

This invention relates to the recovery of para xylene from a xylene richfraction consisting essentially of a complex mixture of xylenes witharomatic and non-aromatic hydrocarbons boiling in the same range as thepara xylene. More particularly, the invention involves the production ofpara xylene from a mixture of nonaromatic petroleum hydrocarbons.

A complex hydrocarbon fraction with which the present invention isconcerned primarily and from which para xylene may be recovered,typically contains only a minor proportion of para xylene. Theproportion of para xylene in such mixtures seldom is more than 30% byvolume and usually is less than about 21% by volume of the hydrocarbonfraction but should be more than about of the xylene present in themixture. The major portion of the mixture comprises aromatichydrocarbons boiling Within 11 F. of the para xylene and including fromat least about 5% up to as much as 20% or more of ethyl benzene based onthe entire hydrocarbon fraction. The ethyl benzene content may be from50 to 100% by volume of the para xylene content. Of these aromatichydrocarbons at least about 50% by Volume of the xylenes in the fractionis meta xylene, with minor amounts of ortho xylene not exceeding about20% by volume. Additionally, the xylene fraction usually contains atleast about 5% and up to 20% or more (based on the entire hydrocarbonfraction) of unsulfonatable hydrocarbons of unknown constitution.generally identied as parafiinic, which may boil as much as 50 F. belowthe para xylene and not more than a'bout20 F. above the para isomer.These paraillnic hydrocarbons may be present in amounts of from 25 to100% by volume based on the para xylene content and include acyclicsaturated hydrocarbons which either boil within the range or formconstant boiling mixtures with ahe xylenes. Examples of such paraiiinichydrocarbons are various isomeric octanes and nonanes. The presence ofcyclic paraiins, i. e., naphthenes boiling from 50 F. below to 20F.above para xylene is not precluded.

An analysis of a xylene fraction typifying the above discussedcomposition is:

Characteristic boiling ranges of xylene fractions with which thisinvention dealsvare from 230 F. to about 300 F., more desirably boilingwithin the range of from 270,to about 300 F. and preferably within therange of from 270 to about 290 F.

The foregoing specific example has the following boiling rangecharacteristics in an ASTM-D-86 distillation:

The recovery of para xylene from such complex mixtures is not simple,since the presence of not only the isomeric xylenes but also paraflnsand aromatics isomeric to the xylenes, particularly of ethyl benzene,complicate and obscure the purication problem. Methods for recoveringpara xylene from its isomers'have been proposed and prior proposals areof two general types, each of which has significant disadvan tages andhigh cost factors. One type of proposal has involved extensive chemicalalteration of one or more of the hydrocarbon components in the xylenesystem to afford elimination and separation of the components. Suchmethods involve relatively expensive chemical conversions with attendantloss and normally require reconversion of the resulting chemicalderivatives back to the desired hydrocarbon with additional loss at thisstage as well as an overall useless consumption of chemical treatingagent. Alternatively, physical methods heretofore proposed haverecognized the complicating and obscure effects of aromatic andnon-aromatic hydrocarbons in the xylene fractionand have attempted tosolve this problem by removal thereof.

Spannagel Patent No. 1,940,065 allegedly recovers para xylene bycrystallization but first purifies the xylene fraction by distilling offany aliphatic hydrocarbons, ethyl benzeneand the like, boiling belowpara and meta xylene," to avoid the complicating'eiects of theseimpurities. In this patent ortho xylene also is removed and anintermediate meta, para xylene cut boiling from 13G-140 C., andevidently free of complieating hydrocarbon impurities, ls utilizedY inthe crystallization step. Thus, in prior processes it appears that therehas been an attempt to avoid unpredictable, obscuring and complicatingeffects of ethyl benzene by removal thereof as well as elimination ofaliphatic hydrocarbon impurities. These purification treatments requireextensive, elaborate and costly equipment particularly in theelimination of ethyl benzene by distillation.

Reference also has been made to the use of technically pure xylene ofcommerce for the separation of ortho, meta and para xylenes. Asdistinguished from crude xylenes the pure xylenes of commerce contain nomore than 3% and usually less than 1% of paraiilns boiling within therange of from 279 to 285 F. Likewise, the ethyl benzene content of purexylenes of commerce sometimes called technically pure is less thanContrary to the apparent beliefs of those skilled in the art, it hasbeen discovered that para xylene can be recovered in relatively goodpurity by crystallization from ortho and meta xylenes in the presence offrom 5 to 20% or more by volume of ethyl benzenes as well as in theadditional presence of from 5 to 20% or more paraiilns boiling Withinthe range of from 230 to 300 F.

The unpredictability of this discovery can be better appreciated when itis noted that these hydrocarbons not only alter the crystallizationtemperature of para xylene by solvent action but that para xylene formsbinary, ternary and quaternary crystals with various of the componentsand that the various components likewise form such complex crystals indeutectic mixtures with each other. The complexity and unpredictabilityof the system is illustrated by the following list of crystal types inthe four-component system-ethyl benzene, ortho, meta, lpara xylene:

'I'he foregoing list of course is an oversimplication, since it ignoresthe obscuring elects of the multi-component parafllnic portion of thexylene fraction.

According to the present invention, in brief, para xylene is separatedfrom the above-described xylene-rich hydrocarbon mixtures by chilling toa temperature of from -75 to 120 F., preferably from 80 F. to -l10 F.and more desirably to a temperature below 80 F. and just above thatrepresentedl by one of the following v equations:

(l) When theortho xylene content is less than about one-half thepercentage of the meta xylene:

where T2V equals minimum temperature in F., X equals per cent paraillnsin feed, Y equals per cent ethyl benzene in feed and Z equals per centortho xylene in feed.

(2) When the ortho xylene content is greater than one-half thepercentage of meta xylene:

Where T2 equals minimum temperature in F., X equals per cent paraflinsin feed, Y equals per cent ethyl benzene in feed and Z equals per centof meta xylene in the feed.

(3) When the ortho xylene content equals onehalf the percentage of metaxylene:

Where T2 equals minimum temperature in F., X equals per cent parafflnsin feed and Y equals per cent ethyl benzene in feed.

In practicing the invention in its preferred embodiment, para xylene isproduced and recovered from non-aromatic petroleum hydrocarbons. Asuitable xylene fraction is obtained by aromatization, preferably by theso-called hydroforming process in which a naphthenic petroleum fractionis aromatized and xylenes are produced. 'Ihis type of process iswell-known in the petroleum industry. However, because the chemistryinvolved and the mixtures obtained are extremely complex, carefulcoordination of feed stocks and hydroforming conditions is necessary toobtain best results and to yield a preferred xylene fraction forrecovery of para xylene in accordance with this invention.

The present invention is particularly adapted to the treatment of anequilibrium xylene mixture from hydroformed non-aromatic petroleumfractions. The term equilibrium xylene mixture" is here utilized todesignate a xylene fraction containing ortho, meta and para xylenes inthe equilibrium proportions resulting from hydroforming or othersuitable aromatization process, that is, in which the relativeproportions are about o:m:p::2:6:2. The additional ethyl benzene andparafns hereinbefore described are also present. Although, the inventionis particularly adapted to the treatment of this specific type ofmixture, it will be understood that the invention is also applicable toother xylene fractions of the compositions hereinbefore described. Toavoid prolixity, the remainder 0f this description will be made withreference to xylene fractions derived from hydroforming operations.V

In order to produce para xylene from nonaromatic petroleum hydrocarbons,proper selection of feed stocks and aromatizing conditions is importantand essential to the most successful practice of the invention.

FEED STOCKS Naphthenic hydrocarbon mixtures from naphthene-typepetroleum crude oils comprise one preferred type of feed stock. vSuchmixtures are normally termed straight run distillates in the petroleumindustry, although other aromatizable hydrocarbons or dstillates may besubstituted therefor. The hydrocarbons present in this preferred feedstock are believed to consist largely of cyclo-aliphatic hydrocarbonswith six carbon atoms in the cyclo-aiphatic ring and with aliphatic sidechains attached to the ring. Some five and seven carbon atomcyclo-aliphatic rings may be present. Both the number of side chains andthe length of each chain attached to the foregoing rings vary among themany compounds normally present in a petroleum hydrocarbon mixture. Ingeneral, these variables are a function of the average molecular weightor, more particularly, the boiling range and distillation -curve .of thepetroleum fraction. A naphthenic hydrocarbon mixture consistingessentially of hydrocarbons having from six to twelve carbon atoms inthe molecule and preferably composed at least predominantly ofhydrocarbons containing from seven to eight carbon atoms at present isregarded as a more desirable feed stock. The fraction selected desirablyshould boil within the range of from about 180 F. to about 420 F. andpreferably from about 180 F. to about 320 F. In some instances an evenmore narrow cut boiling from 230 F. to 275 F. is preferred. Open chainparaillnic hydrocarbon fractions of these boiling ranges are notprecluded.

AROMATIZATION As previously set forth, an initial step in the exemplaryprocess comprises aromatization of the particular petroleum feed stockselected. Where a naphthenic hydrocarbon mixture is utilized, theconversion of hydrocarbons to aromatics is believed to occur bydehydrogenation of the six carbon atom rings from cyclo-aliphatic toaromatic while leaving alkyl groups attached to the residual nucleus.For example:

CH-CHI Ortho dimethyl cyclohexane CH,

CII-CH3 HgC Para dimethyl cyclohexane CHqCH;

CHzCHg Ethyl cyclohexane saturated paramns and naphthenes. The over# allcomplexity of the mixture and the relative proportion of theabove-mentioned non-aromatic components depend upon the effectiveness ofthe particular aromatization process as well as upon the specifichydrocarbon feed stock selected. It is for this reason that a highlynaphthenic hydrocarbon feed stock boiling within the ranges previouslydisclosed are preferred, since the recation products therefrom arebetter adapted to subsequent processing steps involved in the productionof isomeric xylenes. However, it is possible, but less desirable, toobtain operative aromatic fractions from open chain parafiiniclwdrocarbons by known reactions, such as dehydrogenation and cyclizationillustrated by the following reactions:

p-Xylene H CH:

\C/ HIC CH3 HIC C/CH: n/a.

Di-isobutyl l (2, Bdimethyl hexane) Processes for effecting sucharomatization reactions and catalysts therefor are known in thepetroleum art. Likewise, aliphatic olefins are convertible to aromaticsby known cyclization and dehydrogenation reactions similar to theforegoing. 'I'hese various known processes may be utilized within thebroader aspects of this invention and are embraced within the termaromatization as used in the present specification.

The preferred aromatization process knownas hydroforming" ischaracterized by aromatization in the presence of controlled amounts ofhydrogen and a vanadium oxide or `molybdenum oxide catalyst. As anexample of the preferred process, a hydrocarbon feed, such as anaphthenic petroleum distillate, boiling within the range of F. to 320F., and obtained, for instance. by fractional distillation of a crudepetroleum (from Kettleman Hills Oil Field in California) is passed atfrom about 900 F. to about 1200 F., desirably about 1000 F., over avanadium oxide-alumina or molybdenum oxide-alumina catalyst. Space ratedesirably is from 0.1 to about 2.0 volumes of liquid hydrocarbon feedper volume of catalyst per hour, and it is preferred to maintain apartial pressure of hydrogen in the reaction zone Vof from about 30 toabout 300 pounds per square inch. The reaction product from such ahydroforming operation will contain not only the desired xylenes andadditional aromatic hydrocarbon but also aliphatic hydrocarbons boilingover a wide range, including C4 and like materials. Initially,therefore, it' is necessary to recover a xylene fraction from thisreaction mixture.

In the drawing,

Fig. 1 is a schematic flow sheet of a typical process and suitableapparatus for practicing the process of this invention.

Figs. 2 and 3 illustrate graphically the effect of parafns uponcrystallization temperature and recovery of para xylene.

Figs. 4 and 5 illustrate the eiTect of ethyl benzene on crystallizationtemperaturey and recovery of para xylene; and

Fig. 6 reveals the influence of relative proportions of the xyleneisomers on crystallization recovery of para xylene.

Fig. 7 shows the effect of ortho xylene concentration on optimumcrystallization temperatures and at different ratios of ortho to metaxylenes.

Referring to Fig. 1 of the drawing, a naphthenic hydrocarbon feed isintroduced by way of line l to a hydroforming unit and non-aromaticpetroleum hydrocarbons such as the naphthenic petroleum distillateboiling within the range of 18o-320 F., as previously described, isconverted to a complex aromatic hydrocarbon fraction. Desirably theparticular hydroforming operation is that previously described andexemplied as a preferred process. The hydrocarbon eiiiuent iiows by wayof line I2 to a fractionating column I3 where separation is effected. Ashere shown, the fractionation is effected in a single column although amultiplicity of fractionating units may be utilized. C4 and lighterhydrocarbons are taken as overhead through line |4 while Cs, C6 and C7hydrocarbon fractions are removed separately as side streams by way oflines I5, I6 and I1 respectively. C9 and heavier hydrocarbons aredischarged as bottoms by way of line I 8. The xylene-rich hydrocarbonmixture from which para xylene is to be recovered is withdrawn fromfractionating column I3 by way of lziie I9 and ows through cooler 2| tosurge tank The xylene-rich hydrocarbon fraction containing parai'linsand ethyl benzene, as hereinbefore described. flows from surge tank 22to and through the para xylene recovery system. Although not essentialto operability of the process, it will be found highly desirable invarious instances to adjust the ratio of ortho to meta xylene in thisxylene feed stock in order to enhance recovery of the para isomer. Theortho xylene content of a fraction prepared by hydroforming is less thanthe preferred ratio, and as here shown ortho xylene is added to thehydrocarbon mixture in surge tank 22 by line 23, and the ortho to metaxylene ratio is thereby adjusted to approximately 1:2. The blendedhydrocarbon mixture so formed then flows by way of lines 24 and 28through heat exchanger 21 Where the temperature of the mixture isinitially lowered, most desirably by indirect heat exchange with motherliquor from the crystallization operation. This mother liquor flowsthrough inlet and outlet conduits 28 and 29, but for purposes ofsimplicity connections with the mother liquor lines are not shown.

crystal growing tank 3| where it is reduced to crystallizing temperatureby mixing with previously cooled xylene stock. The xylene stock isretained at crystallizing temperature until the desired crystal form isobtained, that is, until shock crystals are largely removed by remeltingand recrystallization or by equilibrium exchange with larger crystalswhich will be retained and recovered satisfactorily in subsequentfiltering operations. Generally, a residence time of about twentyminutes is preferred.

Extremely rapid cooling of the incoming xylene stream adversely effectspara xylene recovery, tends to lower the purity of product, and producesundesirably iine crystals which can be 'separated from the mother liquoronly with great difliculty, if at all. Thus, a cooling rate in the orderof 50 F. a minute in a batch process produces such adverse effects,Whereas a cooling rate through the crystallization temperature range inthe order of 1 to 10 F. a minute gives a good yield of lterable crystalsofrelatively high purity. More desirably, a cooling rate below about 5F. a minute through the crystallization temperature range may beutilized.

Crystallizing temperature is maintained in soaking tank 3| bycirculation of a xylene side stream through chillers by Way of line 32.Thus, circulation pump 33 forces the xylene throughtemperature-controlled chillers 34, 36 and 31 connected in parallel, asshown, by valve-controlled inlet lines 38, 39 and 4I. Desirably,circulation pump 33 is designed and controlled to force the xylenemixture through the chiller It has been found that recovery of paraxylene minutes or more residence time at crystallizingtemperatures. Ashere shown the xylene stream flows into a suitable heat insulatedsoaking or tubes at a suflicient velocity and under adequate pressure tocause turbulent ow. The term turbulent flow here is used in thekcommonly accepted hydraulic sense. Such turbulent flow is adapted toprevent or minimize localized shock cooling of the xylenes at thesurface of the heat exchange tubes in coolers 34, 36 and 31.Additionally, crystal growth and adherence on the walls of such heatexchange tubes is reduced to a minimum by the use of high velocities,especially those exceeding the minimum for turbulent flow. For example,supercooling may be effected in the heat exchange tubes and thesupercooled liquid returned to the crystallization tank before crystalformation is completed. After reduction to a crystallization temperatureat least as low as that to be maintained in soaking tank 3|, the xylenemixture is passed through chiller discharge lines 42, 4'3, 44 and returnheader 46 to the crystal soaking or growing tank. The chilled xylenemixture is dispersed with the crystal slurry in tank 3| and anequilibrium temperature condition is reached therewith.

Any suitable refrigerant is supplied to the chillers by Way of inletheader 25 and outlet 30. Liquefied ethylene, ethane or methane areexamples of suitable refrigerants. As here indicated, temperaturecontrols 35 are provided in the refrigerant discharge line of each ofthe chillers to regulate the flow of refrigerant therethrough.Desirably, these controls are responsive to the temperature of thexylene mixture in discharge lines 42, 43 and 44 respectively.

Upon completion of the crystal growing operation in tank 3|, the slurryof para xylene crystals in the remaining liquid hydrocarbon mixture isconveyed by suitable means, as indicated by line 41, to a crystalseparation and recovery unit. As illustrated herein, crystal separationand purification are eiected by a combination of centrifugal lters andan agitated tank washer. Initially the crystals in slurry from tank 3|are separated in a centrifugal filter 48 at a temperature of from about75 F. to about 120 F., more desirably 80 F. to 110 F., and preferablyfrom 80 F. to T2 as previously defined, and conveyed as indicated byline 48 to crystal washer 5|. Any Suitable washing fiuid may beutilized, such as isopentane, alcohol or the like, but as here shown apara xylene saturated hydrocarbon mixture is -introduced by way ofvalve-controlled line 52 with the slurry and the mixture intimatelycontacted by agitators 53. The resultant slurry flows through outletline 54 to a second centrifugal filter 56. In order to maintain andcontrol the temperature in washer I, a portion of the washing liquid instream 54 is by-passed through valvecontrolled line 51, heater 58 andreturn line 59 to washing tank 5|. Steam or other fluid heating agent issupplied to heater 58 as indicated by inlet and outlet lines 6| and 62.

'I'he crystal slurry from washer 5| is separated in the second stagecentrifugal filter 56, and the purified crystals removed and transferredto melting tank 63 as indicated by line 64. The ltrate from this secondstage separation is discharged by way oi' line 66. 'I'his filtratecomprises a xylene fraction saturated withv respect to para xylene atltration temperature. A portion thereof flows by way o fvalve-controlled line 52 to be utilized as the washing liquid in tank 5|The remainder of the nitrate from unit 56 passes by way of recycle line61 through heat exchanger 21, and preferably is blended with the xylenefeed stock, before it is introduced into soaking tank 3|.

In some instances it will be found desirable to minimize crystalformation in chillers 34, 36 and 31 by recirculating the filtrate ofrecycle line 51 through the heat exchanger tubes 38, 39 and 4| togetherwith, or in lieu of, xylenes from crystal forming tank 3|. A by-passline 61a from recycle line 61 to pump 33 is provided for this purpose.

Purified crystals ofpara xylene in tank 63 are melted and passed tostorage 68 by way of line 89. A portion of the melted stock is by-passedthrough valve-controlled line 1I, heater 12 and return line 13, theheated xylene serving to melt crystals fed to the system. Heat issupplied by hot water or any other suitable fluid introduced through theline 'I4 and discharged through line 15.

'I'he two-stage filtration and crystallization system preferablyisoperated with first-stage filter 48 maintained at a lower temperaturethan second-stage lter 56. A portion of the crystals discharged fromwasher 5| is allowed Ato melt so that the filtrate from unit 56 is paraxylene of the desired purity, thereby furnishing a wash liquid rich inpara xylene by way of valve-controlled line 52 for removing entrainedless-pure mother liquor from the crystals in washer 5|. Temperature insuch a washing operation may be from about +20 F. to +35 F. althoughlower temperatures may be used, depending upon purity and yieldsdesired.

Mother liquor from first stage filter 48 is discharged by way of outletconduit 11, and in the embodiment here illustrated passes tofractionating column 18 wherein an ortho xylene fraction is separated bydistillation.

In this distillation as relatively high purity ortho xylene fraction(for example, 95% or higher) can be obtained by superfractionation,which is a preferred type of operation for the present invention. Theortho xylene is removed from the l0 distillation as a bottoms fractionby way of discharge line 19. A portion of the ortho xylene desirably isrecycled by way of valve-controlled line 23 to feed surge tank 22 in anamount sumcient to adjust the ortho xylene content of the feed aspreviously disclosed herein. The remainder of the ortho xylene flows tostorage by way 'of valve-controlled line 8|. Overhead fromsuperfractionator 18 passes by way of line 82 to'storage 83. Thisoverhead fraction consistsv of a mixture of xylenes, primarily metaxylene with minor amounts of ortho and para xylenes as well as withparafhns and ethyl benzene contained in the original feed stock.

With respect' to the separation of an ortho xylene fraction bydistillation and superfractionation, it should be noted that it will benecessary to maintain the non-aromatic hydrocarbon contentl of thexylene fraction supplied to superfractionator 18 below about 15% byweight. When necessary this initial purification may be eiIected in anysuitable manner as, for example, by an initial extractive distillationof the xylene, or by liquid phase selective solvent extraction or thelike. The superfractionation itself requires a highly eillcientfractionating column. One equivalent to 35 theoretical plates isnecessary for practical operation, more de sirably about 45 andpreferably about 60 theoretical plates are utilized. Reflux ratios ondistillate of, from about 7:1 to about 12:1 have been foundsatisfactory. Very close temperature regulation is important, and thedistillation is so sensitive that control by temperature responsivedevice has been found to give ineiilcient though operable separation. Apreferred method of superfractionation is to operate the fractionatingunit continuously at a given constant feed rate while (1) removingoverhead distillate and bottoms at a constant ratio corresponding to thefeed rate and in a relative proportion such that the desired purity ofthe ortho xylene may be maintained, and (2) maintaining a constantvolume of liquid and still bottoms by controlling the rate of heatinputthereto. Maintenance of the constant volume of bottoms may beeffected, for example, by a constant level control which increases theamount of steam admitted to the still heating unit when the level of thestill bottoms begins to rise, and decreases steam input when the volumeof bottoms begins to drop below the predetermined level. With thebenefit of the foregoing instructions, those skilled in the art can eectsuperfractionation of a mother liquor boiling Within the range of, forexample, 275-295 F. and having a non-aromatic hydrocarbon content ofless than about 15% by weight.

To further illustrate the invention and guide those skilled in the artin the practice thereof, data showing effective recovery of para xylenein the presence of different amounts oi parains and at differenttemperatures are presented graphically in Fig. 2. The feeds B and Creferred to in Fig. 3 had the following composition:

Figs. 2 and 3 illustrate that the presence of paralllns tends todecrease para xylene recoveryat any given' temperature, but that thisdecrease in recovery is avoidable by further reducing crystallizationtemperature 'within the limits and in the manner hereinA disclosed; thatis, by lower- 12 TABLE I Eect of paramns on the recovery of para ing thetemperature 0.3 F. for each per cent of b renin Feedo parafns present.bi hth fr t ith nc P nt `1'igs.4and5estals ee ecso eprese e meA Perma.of ethyl benzene and show that V it tends to de.. g'lginm" 1kg creasep-xylene recovery at any given temperagiggle 53.5 35.4 ture to a greaterextent than do the Paramus 10 Pmmnsmmwmne) fgjg '12 Likewise, the dataillustrate that this decrease in recovery is avoidatble by reditisgcrxstiarlllicondlfin: 50 l d tion temperature wi hin the i an e rsea r.ee manner herein disclosed, that is, by lowering gargifgmfflghogfcmHscrystallization temperature as previously disclosed (4) Cake if dried 2minutes- Crystal- Percent Charge Percent Iizing Cooling mme af p-X PefntParamus Tetrllllrrera- Time ,1.31% Cryigta 1s excglg:

"r, Mm. Mm. 60g.B"--..-- 10.2 -75 20 5 80.0 45,9 -75 2s zo 80.5 43.8-ioo as 1o es um 1oe ao 3o '12 69.5 so g. C"..--- 3a 6 -90 25 2o 74. 753.0 we eo 4o 74.3 63.7 no 1o 77 eze about 1.4 F. for each percentage ofethyl benzene In Tablev II are shown data, on the effects of present 1nthe feed, 4 l t successively increased percentages of ethyl ben. Figs. 6and 7 show the effects of ortho xylene to zene obtained by addition ofethylI benzene to the meta xylene ratio on para xylene recovery andoriginal charge stock. y on optimum crystallization temperature. TheTABLE n data of these two figures are based on compositions containingpara xylene in excess of the xyl- Eect of ethyl benzene on recovery 0fpara ene eutectics. Thus, when the ortho-meta xylene xylene ratio isless than one-half, c:rytaltlizoaicnlpteirn Chage stoctmth lbenz Irature should be decrease a ou or ereen e y ene..- .o Egon per cent ofortho xylene present. When 21?- ?nglf"- l the ratio of ortho to metaxylene is greater than 15g; g ggg; mi one-half, the optimumcrystallization temperature condliitiri; "t' ".1145 for any given feedcontaining para xylene in exafg@ c0- uenep yopp est over eed cess of theeutectic proportion should be increased lciliilmcolfgho cm' Hg about 1.3F. for each per cent of ortho xylene 45 4) Cake alf dried2minutespresent in excess of 33, based on the xylenes.

Again, when the ortho to meta xylene ratio is the Per Cent ryscalliz-Cooling 'rime at Pez'cfellg Per Cent optimumv 1:2, then the mostdesirable crystalliza- Bglg'e 13g l Time gstp y1 Il-Xylenei tiontemperature is 84.5 F. decreased by the P crystals scovare correctionfactors previously disclosed for para- 50 o in and ethyl benzenecontents only. 14.0 1'75 35 1o mo 4e s An exemplary process was carriedout and data 28.4 -75 3o an 74.3 3813 obtained in a simplied apparatusconsisting of jg :gg gg a fritted glass filter surroundedby a coolingbath 14.0 9o lao so 76.8 5&8 to maintain the filtration at specifiedtemperatures. I@ 32 Crystals were ltered from the mother liquor by 38.6-oo 2o 1o 7z`0 ae's applying vacuum, and the crystal cake of para 22:13g gg 1g g'lig ggg xylene was air-dried for a measured time interval.38. 6 los 25 5 ca -2 472 o Crystals were weighed and purity determinedby the freezing point method. To regulate the sweato0 A second series ofexemplary runs was made ing of the crystal cake and eliminate iceformawith centrifugal separation of para xylene crystion on the lter,the air used for drying was rst tals. The filtration was effected inequipment chilled with an alcohol solid CO2 bath. In these consisting ofa perforated basket centrifuge runs the percentage of paraillns wasvaried by lined with muslin. An agitated chilling vessel adding theunsulfonatable residue (that is, the was provided for cooling the feedstock by inparaiiinic hydrocarbons) of a xylene fraction I ternalrefrigeration by direct addition of Dry formed by hydroforming apetroleum hydrocarbon Ice. In addition to the mixing effected byevapofraction as previously described herein. By utilrated CO2,mechanical agitation was utilized to izing this particular mixture ofparaillns, repreaid in controlling the temperature of the chargesentative results were obtained without the necesstock and in reducingagglomeration of the sity of identifying the exact composition andprosolid CO2. portions of the different parafilnic components. The paraxylene crystal slurry was fed by grav- 'Iables I and II give the resultsof representative ity into the centrifugal filter, and a pump was .runsmade in the foregoing manner, Table I showprovided for recirculation ofthe cooled mother lng the eirect of the parans:

liquor from the filter back to the agitating ves?.

13 sel. The centrifugal pump, agitator and pipe lines were suitablyinsulated to maintain low temperatures. Means for' measuringtemperatures in the agitator and of the inlet and outlet of thecentrifuge were provided. In operation, the whole system was graduallycooled to the desired crystallization temperature by addition of solidCO: to the agitator and continuous recirculation of the xylene motherliquor through the agitator and centrifugal filter. Purification of thecrystals in situ was effected in two stages; first, extraction ofimpurities by circulation of the mother liquor through the filter cakefor a substantial period after crystallization temperature is reached;and secondly, by drawing on the mother liquor and allowing the filtercake to rise in temperature suiliciently to sweat out hydrocarbonimpurities while continuing operation of the centrifuge to removeliquefied impurities so released. Data from these runs are given inTable III:

TABLE DI Charge Stock:

Per cont Ethyl benzene i4. Per cent o-Xylene ,-L d l Per cent m-Xylene49. 3 Per cent Xylene 19. l Per cont arailins 1l. 5

Final Cen- Recovery of Crystalliz- R. P. M. of Purity of ing Temp.ltfgil centrifuge p-Xylene lrlge F. Min. Percent Per cent YV14... liquidfrom the crystal phase. withdrawing the washed crystal phase as aproduct, conducting zone during the succeeding cycle and utilizing theremainder as the specified wash liquid to wash the crystal phaseproduced in the succeeding cycle of operation. 4`

2. A cyclic process for recovering paraxylene from a xylene richfraction of catalytically reformed naphtha boiling in the range about270- 300 F., each cycle comprising the steps of`cooling said fraction toa temperature in the range 75 F. to 120 F. for a time sufficient tocause the formation' of a solid crystalphase comprising paraxylene,ltering the cooled mixture to separate the crystalline phase and amother liquor and withdrawing the mother liquor as a product, washingthe crystal phase with a wash liquid having a, paraxylene contentsubstantially greater than that of the mother liquor, filtering the washliquid from the crystal phase, withdrawing the washed crystal phase as aproduct, conducting the washing and second mentioned filtering steps attemperatures substantially above the temperature of the cooling zonesuch that a subtantial portion of the crystal phase is melted duringsaid steps, returning a portion of the filtrate from the secondmentioned filtering step to the l Equipment was cork insulated for thisand ensuing runs. Low

recovery due to increased speed of centrifuge.

I Some mechanical loss of product from centrifuge.

l Centrifuga modilled to permit measurement of R. P. M. For ggg'zunptlli pseed of the final centrifuge period was increased to 4 Thisperiod was reduced by increasing the air flow through the ntl'lfllge.

It is readily apparent from the foregoing description that variousmodifications of the process can be made within the spirit of thepresent invention and the scope of the appended claims. For the sake ofsimplicity and clarity, apparatus has not been shown in detail in thedrawing but is illustrated only as to major unit operations in theprocess. Many detailed pumps, valves, condensers, heat exchangers,temperature controls and the like have been omitted, since any suitableform of apparatus incorporating these features can be supplied inobvious manner by those skilled in the art.

I claim:

l. A cyclic process for recovering paraxylene from a hydrocarbon liquidcomprising substantial amounts of orthoxylene, metaxylene andparaxylene, each cycle comprising the steps of cooling said liquid to a.temperature in the range 75 F. to 120 F. for a time suflicient to causethe formation of a solid crystal phase comprising paraxylene, filteringthe cooled mixture to separate the crystalline phase and a mother liquorand withdrawing the mother liquor as a product, washing the crystalphase with a wash liquid having a paraxylene content substantiallygreater than that of the mother liquor, filtering the wash cooling zoneduring the succeeding cycle and utilizing the remainder as the specifiedwash liquid to wash the crystal phase produced in the succeeding cycleof operation.

3. The method of separating paraxylene from catalytically reformednaphtha which comprises fractionally distilling said naphtha to separatea xylene rich fraction having a boiling range from about 270 F. to about300 F., passing said fraction into a cooling zone and cooling it to atemperature in the range F. tof-120 F. to separate a solid crystallinephase comprising paraxylene and a mother liquor phase, filtering thecooled fraction without appreciably raising its temperature to separatethe crystalline phase from the mother liquor, withdrawing the motherliquor as a product, washing the crystalline phase with a wash liquidhaving a paraxylene content substantially higher than the mother liquor,fil-` tering the mixture of wash liquid and crystalline phase at atemperature substantially above that at which the crystallization iseffected such that a substantial .portion of the crystalline phase ismelted, withdrawing the washed crystalline phase as a product, returninga portion of the liquid eilluent from the second mentioned filtration`to the cooling zone and utilizing the remainder as the Wash liquid inwashing further quantities of separated crystalline phase.

4. The method as defined in claim 3, characterized by the further stepsof fractionally distilling the mother liquor to separate metaxylene asthe overhead fraction and a liquid rich in orthoxylene as the kettleproduct and introducing into the cooling zone together with the xylenerich fraction a portion of said kettle product to raise the ratio oforthoxylene to metaxylene in the resultant mixture.

5. A cyclic process for recovering paraxylene from a hydrocarbon liquidcomprising substantial amounts of orthoxylene, metaxylene andparaxylene. each cycle comprising the steps of cooling ing a paraxylenecontent substantially greater than that of the mother liquor, filteringthe wash liquid from the crystal phase, withdrawing'the I0 washedcrystal phase as a product, conducting the washing and second mentionedllltering steps at temperatures substantially above the temperature ofthe cooling zone such that a substantial portion of the crystal phase ismelted during said steps, returning a portion of the ltrate Iromthesecond mentioned ltering step to the .cooling 16 zoneduring, a.succeeding cycle and utilizing the remainder as the major component ofthe specied wash liquid to wash the crystal phase produced in thesucceeding cycle of operation. y JEROME HOWARD ARIIIOLD.A

` REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number` Name Date 1,940,065 Spannagei Dec. 19,1933 2,383,174 Weir Aug. 21, 1945 2.398.526 Greenburg Apr. 16, 19462,400,883 Keeling et al May 28, 1946 2.435,79?

MCArdle et al. Feb. 10. 191:8

1. A CYCLIC PROCESS FOR RECOVERING PARAXYLENE FROM A HYDROCARBON LIQUIDCOMPRISING THE STEPS OF TIAL AMOUNTS OF ORTHOXYLENE, METAXYLENE ANDPARAXYLENE, EACH CYCLE COMPRISING THE STEPS OF COOLING SAID LIQUID TO ATEMPERATURE IN THE RANGE -75* F. TO -120* F. FOR A TIME SUFFICIENT TOCAUSE THE FORMATION OF A SOLID CRYSTAL PHASE COMPRISING PARAXYLENE,FILTERING THE COOLED MIXTURE TO SEPARATE THE CRYSTALLINE PHASE AND AMOTHER LIQUOR AND WITHDRAWING THE MOTHER LIQUOR AS A PRODUCT, WASHINGTHE CRYSTAL PHASE WITH A WASH LIQUID HAVING A PARAXYLENE CONTENTSUBSTANTIALLY GREATER THAN THAT OF THE MOTHER LIQUOR, FILTERING THE WASHLIQUID FROM THE CRYSTAL PHASE, WITHDRAWING THE WASHED CRYSTAL PHASE AS APRODUCT, CONDUCTING THE WASHING AND SECOND MENTIONED FILTERING STEPS ATTEMPERATURES SUBSTANTIALLY ABOVE THE TEMPERATURE OF THE COOLING ZONESUCH THAT A SUBSTANTIAL PORTION OF THE CRYSTAL PHASE IS MELTED DURINGSAID STEPS, RETURNING A PORTION OF THE FILTRATE FROM THE SECONDMENTIONED FILTERING STEP TO THE COOLING ZONE DURING THE SUCCEEDING CYCLEAND UTILIZING THE REMAINDER AS THE SPECIFIED WASH LIQUID TO WASH THECRYSTAL PHASE PRODUCED IN THE SUCCEEDING CYCLE OF OPERATION.